Thermomechanical behaviour in Ni-Ti based melt-spun ribbons

The temperature induced martensitic transformation under several tensile stresses (thermomechanical cycling) has been studied in an equiatomic NiTi and in a Ni25Ti50Cu25 melt-spun ribbons obtained by planar flow casting. Both ribbons have been characterized by DSC, TEM and X-ray diffraction; the Cu-containing ribbon, being almost fully amorphous in the as-spun state, has been crystallized before thermomechanical cycling. The NiTi ribbon shows, on cooling, the P -->R --> B19 transformation sequence, whereas the Ni25Ti50Cu25 transforms to the B19 martensite structure. The strain-temperature curves have been analyzed as a function of the applied load up to 60 MPa. A good two-way shape memory effect is obtained in the NiTi ribbon, but not in the Ni25Ti50Cu25. This ribbon, although presents a net shape memory effect, shows an anomalous effect at the beginning of the cooling branch in the strain-temperature curves, consisting in a contraction prior to the elongation. The anomaly is also seen on heating, producing some discontinuities at the end of the heating branch. The possible origin of this effect is discussed. Ageing effects at 500 degreesC on the thermally induced martensitic transformations have been previously investigated, in order to complete the ageing effects knowledge, thermomechanical cycling series have been performed before and after ageing at 500 degreesC.